Coherent hexagonal platinum skin on nickel nanocrystals for enhanced hydrogen evolution activity

Metastable noble metal nanocrystals may exhibit distinctive catalytic properties to address the sluggish kinetics of many important processes, including the hydrogen evolution reaction under alkaline conditions for water-electrolysis hydrogen production. However, the exploration of metastable noble metal nanocrystals is still in its infancy and suffers from a lack of sufficient synthesis and electronic engineering strategies to fully stimulate their potential in catalysis. In this paper, we report a synthesis of metastable hexagonal Pt nanostructures by coherent growth on 3d transition metal nanocrystals such as Ni without involving galvanic replacement reaction, which expands the frontier of the phase-replication synthesis. Unlike noble metal substrates, the 3d transition metal substrate owns more crystal phases and lower cost and endows the hexagonal Pt skin with substantial compressive strains and programmable charge density, making the electronic properties particularly preferred for the alkaline hydrogen evolution reaction. The energy barriers are greatly reduced, pushing the activity to 133 mA cmgeo–2 and 17.4 mA μgPt–1 at –70 mV with 1.5 µg of Pt in 1 M KOH. Our strategy paves the way for metastable noble metal catalysts with tailored electronic properties for highly efficient and cost-effective energy conversion.

Fourier diffractograms corresponding to the zones labeled in (c). The XRD pattern confirms that the hcp-Ni@Pt core-shell nanobranches are successfully transformed into the fcc phase by the thermal treatment. STEM image shows no obvious morphological change during the phase transformation. A high-contrast skin can be observed at the edge of the nanobranches, corresponding to Pt atoms, which confirms that the core-shell nanostructure has been retained during the thermal treatment. From the HRTEM images, we can clearly observe the fcc zones, although there are minor hcp zones remaining in the nanobranches. The XRD pattern as a whole-sample-based analysis confirms that the fcc phase is the major phase in the sample.

Evaluation of specific activities of the hcp-Ni@Pt nL nanobranches in alkaline HER.
The specific activity (SA) is defined as the current density normalized to the electrochemically active surface area (ECSA) of the catalyst. The ECSA of Pt-based catalysts can be usually obtained by underpotential deposition of hydrogen in the potential range of 0.05-0.4 V vs. RHE. However, the Ni cores in the hcp-Ni@PtnL nanobranches are prone to oxidation at positive potentials, albeit with the protection of surface Pt. Therefore, it is difficult to obtain the ECSA values, and thus the specific activity, of the hcp-Ni@PtnL nanobranches. To overcome the difficulty in directly measuring the ECSA experimentally, we here adopt two alternative ways to estimate the ECSAs, and thus the specific catalytic activities, as follows.
(1) We assume Pt is grown on the Ni substrate as a uniform thin layer (atomic layer number, n).
For this specific structure, the fraction of Pt atoms on the surface is 1/n. Given a constant mass of Pt, the number of Pt atoms on the surface (thus the specific surface area) is proportional to 1/n: Because the specific activity (SA) is related to the mass activity (MA) by the following equation: the specific activity can be derived as: Therefore, the value obtained by multiplying the mass activity (MA) of the hcp-Ni@PtnL nanobranches by the atomic layer number of the Pt skin (n) can be used as a measure of their specific activity (SA), with MA and n easily obtainable by electrochemical experiments and Cs-corrected HAADF-STEM imaging, respectively. The unit of the specific activity is mA·layer·µg -1 , which is a variant of the traditional unit of mA cm -2 . Because the evaluation of the specific activity by this method involves few assumptions, we adopted this approach in Fig. 3d to reliably reflect the intrinsic HER activities of the hcp-Ni@Pt-skin core-shell nanobranches to be correlated to the atomic layer number of Pt in the nanobranches. In (b), the area was occupied by 3 blue atoms (6 at the corners × 1/3 + 1 at the center) and 3 green atoms, thus 6 in total.
The area of (b) can be calculated as: The area of occupied by each atom (A0) can be calculated as: The specific surface area is This result, thus, quantifies the k value in eq. 1, i.e., k = 222.8 m 2 g -1 .
The ECSAs and specific activities of the hcp-Ni@PtnL nanobranches in the HER can be calculated by eq. 4, as summarized in Supplementary Table 2. The current densities were normalized to the geometric area of the electrodes. The plots were derived from the LSV curves in Figure 4b (Experimental conditions: 1.5 µg Pt, 1 M KOH, 90% iR compensation). We consider an HER process containing two electron-transfer steps (number of transferred electrons, n = 2). The Butler-Volmer equation is described as

Supplementary
where j is the current density, j0 is the exchange current density, α is the transfer   The error bars indicate the standard deviations of the pH values from 10 parallel measurements.